Sublimation getter pump employing a consumable getter source element heated by radiation



Feb. 6, 1968 w. A. LLOYD 3,367,564

SUBLIMATION GETTER PUMP EMPLOYING A CONSUMABLE GETTER SOURCE ELEMENTHEATED BY RADIATION Filed May 16, 1966 5 Sheets-Sheet l.

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23 av. ac. CURRENT ,NVENTOR SUPPLY SENSOR BYWILLI MA.LLOYD Et -H RNEYFeb. 6, 1968 w. A. LLOYD SUBLIMATION GETTER PUMP EMPLOYING A CONSUMABLE(BETTER SOURCE ELEMENT HEATED BY RADIATION Filed May 16, 1966 5Sheets-Sheet 2 FlG.4

INVENTOR. ,WIL IAM A.LLOYD Call ORNEY 3,367,564 GETTER Feb. 6, 1968 w.A. LLOYD SUBLIMATION GETTER PUMP EMPLOYING A CONSUMABLE SOURCE ELEMENTHEATED BY RADIATION Filed May 16, 1966 5 Sheets-Sheet 3 FIG.5

INVENTOR BY WILL A A. LLOYD WJG'K NEY United States Patent 3,367,564SUBLIMATION GETTER PUMP EMPLOYING A CONSUMABLE GETTER SOURCE ELEMENTHEATED BY RADIATION William A. Lloyd, San Jose, Calif., assignor toVarian Associates, Palo Alto, Calif., a corporation of California FiledMay 16, 1966, Ser. No. 550,383

Claims. (Cl. 230-69) simulator chambers and vacuum metallurgial refiningsystems.

Heretofore, high speed, high capacity sublimation vacuum pumps have beenused wherein a consumable getter source was heated to sublimationtemperatures by direct electron bombardment from a plurality of shieldedfilamentary emitters positioned around the getter source element,typically a rod of titanium. The problems with such an arrangement arethat the emitter shields, typically operating at or near cathodepotential, are relatively cool compared to the operating temperature ofthe sublimation region of the consumable rod. Therefore, the sublimedgetter material tends to condense on the emitter shields and otherelements of the relatively open structure producing 'a substantial buildup of getter material. This build up is wasteful of getter materialwhich flakes off in use and oftentimes produces undesired arcs withresultant failure of the sublimator. Moreover, the consumable rod formsthe anode for the bombarding electron guns. The rod is not alwaysequally heated over its exposed bombarded surface area. As a consequencethe end of the rod, forming the anode, takes different random timevarying contours which are dependent upon its history of heating andsublimation. These different time varying anode contours serve toeffectively time vary the perveance of the various electron guns indifferent ways. As a result it is not possible to precisely control thesublimaton rate by controlling the current or voltage supplied to thesublimator or by controlling the rate of advance of the rod in anypractical way.

In the present invention, the consumable getter element is advanced intoa high temperature zone which is heated to getter sublimationtemperatures by means of a thermal radiator. Thethermal radiator isheated by electron bombardment from one or more electron emitters whichare shielded from the getter source element by means of the radiatordisposed therebetween. The advantages of this arrangement, over theprior electron bombarded sublimator, are that the radiator, since itoperates above sublimation temperatures, does not collect I the sublimedmaterial, thereby obtaining more efficient use of the sublimed gettermaterial and avoiding undesired flaking of getter material in thesublimator. In addition, since the parameters of the electron gun orguns remain fixed, the radiator may be easily held at predeterminedtemperature and the sublimation rate precisely controlled by controllingthe rate at which the getter source element is advanced into thesublimation zone.

The principal object of the present invention is the provision of animproved getter sublimation pump.

One feature of the present invention is the provision of a thermalradiator, heated above getter sublimation temperature by electronbombardment and disposed adjacent the getter source element to besublimed, for defining a 3,367,564 Patented Feb. 6, 1968 sublimationtemperature zone into which the getter source element is advanced forsublimation, whereby the radiator which may be used to shield theelectron gun does not collect sublimed getter material.

Another feature of the present invention is the same as the precedingfeature wherein the radiator surrounds the getter source element and isdisposed intermediate the source of bombarding electrons and the gettersource element.

Another feature of the present invention is the provision of means forautomatically advancing the source of getter material into thesublimation Zone at a controlled rate.

Another feature of the present invention is the same as the precedingfeature wherein the getter source material is advanced into thesublimation zone at a rate controlled in response to a measure of thegas pressure in the system being pumped.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram, partly in block diagram form, of a vacuumsystem employing a sublimation vacuum pump of the present invention,

FIG. 2 is an enlarged longitudinal sectional view of a portion of thestructure of FIG. 1 delineated by line 2-2.

FIG. 3 is a transverse view, partly broken away, of the structure ofFIG. 2 taken along line 3-3 in the direction of the arrows.

FIG. 4 is a schematic longitudinal sectional view of a similar region tothat of FIG. 2 depicting an alternative embodiment of the presentinvention.

FIG. 5 is an enlarged longitudinal sectional view of a portion of thestructure of FIG. 1 delineated by line 5-5, and

FIG. 6 is transverse sectional view of the structure of FIG. 5 takenalong line 6-6 in the direction of the arrows.

Referring now to FIG. 1 there is shown a portion of a vacuum chamber 1such as, for example, a space simulation chamber 40 feet in diameter andof 50,000 cubic feet volume which is to be evacuated to a pressure onthe order of 10- to 10 torr or less. A vacuum pump assembly 2 isconnected into the vacuum chamber 1 via an elbow connection 3, as of 60in diameter, communicating with the chamber 1 through an exhaust port 4.Liquid nitrogen cooled chevron type baffles 5 are disposed across theexhaust port 4 and also line the interior surfaces of the elbow 3 forproviding surface film getter pumping regions, more fully describedbelow.

The vacuum pump assembly 2 comprises a convention- 211 high capacitygetter ion pump structure 6 of the type described and claimed in US.Patent 2,993,638, issued July 25, 1961, and assigned to the sameassignee as the present invention. Briefly, the getter ion pump 6includes a plurality of rectangular pumping chambers 7 communicatingwith the elbow 3. Each pumping chamber 7 includes a multiple coldcathode magnetically confined glow discharge anode array 8 disposedbetween a pair of cold cathode plates 9. The plates 9 are made of agetter material such as titanium. C-shaped magnets 11 are disposedaround each of the pumping chambers 7 for providing a magnetic fieldwhich threads through the anode cells. The getter ion pump 6 provides inexcess of 2000 liters/ second of pumping speed down to pressures of 10*to 10- torr.

A sublimation getter vacuum pump 12 is coaxially disposed of the getterion pump 6 and is carried from a flange assembly 13 closing off thelower end of the elbow 3. The structure of the sublimation pump 12 willbe more fully described below with regard to FIGS. 2-6. Briefly, thesublimation pump 12 comprises an open ended tubular thermal radiator 14heated to operating temperature in excess of 2000 C. by electronbombardment with 6 kv. electrons obtained from a pair of filamentarythermionic emitters 15 disposed around the outside of the tubularradiator 14. A cylindrical radiation shield 16 surrounds the emitters15.

The tubular thermal radiator 14 defines a sublimation zone 17 within itsinterior which operates at sublimation temperatures of about 2000 C. Arod of getter material 18 is coaxially disposed of the thermal radiator14. The upper end of the getter rod is advanced into the sublimationzone 17 from the bottom via a closed loop drive chain 19 linked to therod 18. The drive chain 19 is driven via a drive sprocket 21 actuated bya rotary feed through, not shown in FIG. 1.

The sublimed getter material effuses through the open end of the tubularradiator 14 in a cone about 90 to 120 wide and is collected on thesurfaces of the liquid nitrogen cooled baffles which face thesublimation zone. The deposited getter film serves to getter (pump)chemically active gaseous constituents of the atmosphere inside thechamber 1 which flow or diffuse into the elbow 3. The getter ion pump 6serves to pump the non-chemically active gases as well as the chemicallyactive gases. The pumping speed of the sublimation pump 12, forchemically active gases such as nitrogen, is about 45 liters/second persquare inch of getter film which is deposited on liquid nitrogen cooledsurfaces and about 15 liters/second per square inch for getter filmdeposited on room temperature surfaces. The sublimation pump provides apumping speed in excess of 120,000 liters/second when the pressure issufficiently high such that the getter film is used for getter-ing asrapidly as it is deposited. The getter rod 18 is about 1 /8" in diameterand 26" long and contains about 2000 grams of titanium which is sublimedat a maximum rate of 1.3 grams per hour.

The thermal radiator 14 forms the anode for the thermionic emitters 15.A high voltage supply 22, as of 6 kv., has its positive terminalconnected to the thermal radiator 14. A current sensor 23 is connectedin the anode circuit to ground to derive an output proportional to theanode current. The output of the current sensor is fed to a filamentsupply 24 which is connected across the thermionic emitters 15 forcontrolling the heating current to the filamentary thermionic emitters15. The output of the current sensor 23 is adjusted to maintain thetemperature of the thermal radiator 14 and thus the sublimation zone 17at some predetermined temperature such as 2000 C. Either one of thefilamentary emitters is sulficient to heat the radiator 14 to itsoperating temperature. Upon failure of either one of the emitters 15,the remaining emitter 15 takes over the heating function.

The rate at which getter material is sublimed is determined by the rateat which the getter rod 18 is advanced into the sublimation zone 17. Astepping motor 25, disposed outside of the vacuum envelope, serves toadvance the rod 18 through the intermediary of a mechanical drivemechanism, which includes a shaft 26, and a Wobble stick rotaryfeedthrough, not shown. The rotary feedthrough drives the chain 19through rotation of the drive sprocket 21. Each step of the steppingmotor advances the rod by about 0.002". A pulser 27 supplies the drivepulses to the stepping motor 25 in response to an input derived fromeither a timer and sequencer 28 or a pulse rate controller 29. Switches31 and 32 are provided to interconnect the pulser 27 to either one ofthese pulse rate control devices 28 or 29, respectively.

When switch 32 is closed the timer and sequencer automatically suppliescommand signals to the pulser 27 to cause the stepping motor to advancethe rod 18 at some predetermined fixed rate such as, for example, at arate to sublime 1.3 grams/hour. With each 0.002" advance,

the rod sublimes about 0.1 of a gram. Thus, for the above 1.3 g./hr.rate, the timer 28 commands pulses from the pulser 27 at the rate of 13pulses per hour.

When the switch 31 is closed and switch 32 opened the rod 18 isautomatically advanced at a rate proportional to the gas pressure withinthe system, thereby obtaining optimum use of the available gettermaterial in the rod 18. A vacuum gauge 33 senses the pressure in thevacuum chamber 1 or elbow 3 via pressure sensor 34. The output of thevacuum gauge 33 is fed to the pulse rate control 29 for controlling thepulse rate output of the pulser 27 and thus the rate of advance of therod 18 into the sublimation zone 17. The output of the pulse ratecontrol is proportional to the gas pressure within the chamber 1 beingevacuated. Thus, at relatively high pressures of 10- torr, the rod 18 isadvanced at a maximum rate, whereas at lower pressures such as 10" torrthe rod 18 is advanced at lower rates such as, for example, 0.002" perhour.

A counter 35 is coupled to the output of the stepping motor 25 forcounting the number of steps through which the rod 18 has been advanced.Thus the output reading on the counter 35 is a measure of the amount ofthe rod 18 that remains unconsumed. At some predetermined number ofcounts, corresponding to only an hour or two of remaining gettermaterial, the counter feeds an output to a warning alarm 36 to sound thealarm and warn the operator in suflicient time to replace the rod 18. Inthe event the operator takes no remedial action, the counter, at somegreater number of counts corresponding to an exhausted supply of gettermaterial, feeds an output to a shut down circuit 37 which deactivatesthe pulser 27 to prevent possible overrun damage to the sublimation pump12.

Referring now to FIGS. 2 and 3 there is shown, in greater detail, thestructure of the dispensing head portion of the sublimation pump 12. Thetubular thermal radiator 14 is, for example, a 2% long by 1% insidediameter, V thick wall refractory metal material such as tantalum,molybdenum or tungsten. The tube 14 is flanged at its ends for strength.The radiator tube 14 is carried from a coaxially aligned tubular sleeve41, as of 1.5" I.D., Via four axially directed support legs 42, as oftantalum ribbon 0.010" by A" cross section and 1 /2" long, which arespot welded to the radiator 14 and sleeve 41. A tantalum bearing sleeve43, as of 1 I.D., is coaxially carried of the sleeve 41 for providing anupper sliding bearing support for the getter rod 18 which passes throughthe sleeve 43.

The pair of filamentary emitters 15, as of tungsten wire, surround themidsection of the tubular radiator 14 and are each supported from threeaxially directed separate support legs 44, as of 0.090" diametertantalum wire. The support legs 44, which are connected to the filaments15 intermediate the ends of the filaments 15, are supported at theirbase ends from ceramic insulator assemblies 45.

A double walled radiation shield assembly 46 is formed by a pair of thinwalled radially spaced cylinders 47 and 48, as of 0.020 thick tantalum.The outer shield 48 is cup shaped with the bottom 49 of the cup 48 beingapertured to accommodate the rod 18. The inner shield 47 is supportedfrom the bottom 49 of the cup via support tabs 51 spot Welded to the cup47 and shield 46. The radiation shields 46 serve to reflect heat fromthe emitters 15 and radiator 14 back to the radiator 14. The outercup-shaped shield 48 is carried by its lip 52 from a cup-shaped liquidcooled jacket 53 as of thick walled copper. An annular coolant channel54 is provided in the base of the cup-shaped jacket 53 for cooling. Apair of axially directed coolant pipes 55 connect to the channel 54. Adisk shaped radiation shield 56, as of a double thickness of 0.020 thicksheet tantalum, closes off the upper end of the cup-shaped coolingjacket 43. The shield 56 is centrally apertured to accommoda te thetubular radiator 14. A tubular support 57, as of A1" thick wall 6" OD.stainless steel supports the jacket 53 from a demountable vacuum tightmounting flange assembly 58 (see FIGS. 5 and 6). The tubular radiator 14is carried from the bottom side of the cupshaped cooling jacket 53 viathree axially directed ceramic insulator assemblies 59 capable ofholding off the 6 kv. applied between the radiator 14 and the groundedjacket 53 and emitters 15. One of the terminal emitter support legs 44is connected to the grounded jacket 53 and the other terminal leg 44 isinsulated from the jacket 53.

The upper end of the getter rod 18, where it enters the sublimation zone17, takes on a cone shape in use due to sublimation of the rod 18. Thesublimed getter material effuses out through the open end of theradiator 14 into a cone pattern about 90 to 120 in width. In someapplications of the sublimator 12, it is desirable to shape the effusioncone pattern to reduce effusion along the longitudinal axis of thesublimator 12. In these cases a director assembly 61, such as, forexample, a spiral filament of 0.060" diameter tantalum wire wound into acone shape with 0.080 center to center spacing of the wire, is providedover the end of the radiator 14 to direct the effusing getter materialaway from the longitudinal ax1s.

Referring now to FIG. 4 there is shown an alternative embodiment of thethermal radiator 14' wherein the thermionic emitters 15' are locatedinside the tubular thermal radiator 14' for bombarding the radiator fromthe inside. The getter material, which is to be sublimed, is formed intoa relatively thick walled tube 18'. The sublimation zone 17' is locatedin the region surrounding the radiator 14. The tubular rod of gettermaterial 18 enters the sublimation zone 17' from one end and thesublimed getter material effuses away from the zone 17. This design hasthe advantages of directing the sublimed material away from thelongitudinal axis of the sublimator and of reducing the thickness ofgetter rod in the sublimation zone to facilitate sublimation. A doubleradiation shield 56' closes off the end of the tubular radiator 14' forreducing unwanted thermal radiation.

Referring now to FIGS. 5 and 6 there is shown the support and actuatorstructure for the sublimator of the present invention. The getter rod 18is supported at its lower end from a carriage 65 via a high voltageinsulator 66. The carriage 65 includes an upper flange 67 which isnotched at 68 to ride axially along a pair of guide rods 69 as ofdiameter stainless steel rods supported at their ends from the sidewalls of the tubular support 57 via support arms 71.

The drive chain 19 is pinned to the carriage 65and is driven via thedrive sprocket 21 which is turned by a rotary feedthrough 72. An idlersprocket 73 is supported at the upper end of the sublimator 12 from anaxle connected to the support tube 57. The pair of fluid coolant tubes55, which connect to the fluid cooled jacket 53, enter the tubularsupport 57 at the lower end and pass axially thereof to the jacket 53. Ahigh voltage anode lead 75 is connected at its upper end to the radiator14 via bearing sleeve 43 and is held away from the tubular support 57via stand off insulators 76. The lower end of the lead 75 is connectedto the high voltage power supply 22 via high voltage feedthroughassembly 77. A similar lead and feedthrough assembly 78 provides theoperating voltage and current for the filamentary emitters 15. The lowerend of the tubular support 57 is closed off by a cover plate 79.

The sublimator pump 12 of the present invention requires about 5 kw. ofpower and sublimes up to 1.3 grams of titanium per hour and provides anoperating life in excess of 5000 hours. It has the advantage over priorelectron bombarded titanium rod devices in that the rod is operated atthe same potential as nearby elements such as the radiator and itssleeves and bearing surfaces such that any unwanted accumulation ofsublimed getter material will not bridge between elements at differentpotentials to produce a failure of the device. Also the radiator 14shields the emitters 15 and other elements from sublimed getter materialwhereby wasteful and unwanted accumulations of getter materials are notcollected on the sublimator structure.

In a preferred embodiment of the present invention the getter rod 18 isformed of a relatively thin walled tube as of 0.010" thick wall titaniumfilled with titanium pellets. The pellets may be spherical, cubic, orother shapes to provide increased surface area to facilitate sublimationand to reduce thermal conduction down the length of the composite getterrod 18. By reducing thermal conduction along the rod 18 the sublimingregion of the rod 18 is more narrowly defined to prevent unwantedsublimation and loss of thermal energy. The pellet-filled rod forms thesubject matter of and is claimed in copending US. application 552,374,filed May 16, 1966 and assigned to the same assignee as the presentinvention.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A sublimation vacuum pump apparatus of the type having a source ofgetter material which is sublimed onto interior surfaces of a vacuumsystem for gettering and thus pumping gases within the system to beevacuated including, means for forming a thermal radiator disposedadjacent the source of getter material for producing a sublimation zonehaving a temperature in operation above the sublimation temperature ofthe getter material for subliming the getter material in said zone, andmeans for directing a stream of electrons onto said radiator means forheating said radiator to its operating temperature.

2. The apparatus of claim 1 including, means for advancing the gettermaterial into said sublimation Zone as the getter material is sublimed.

3. The apparatus of claim 2 wherein said electron stream directing meansdirects the electron stream against a side of said radiator means remotefrom that portion of said sublimation zone into which the gettermaterial is advanced for sublimation, thereby shielding the source ofelectrons from the sublimed getter material.

4. The apparatus of claim 3 wherein said radiator means is disposedaround the outside of that portion of said sublimation zone into whichthe getter material is advanced.

5. The apparatus of claim 4 wherein said radiator means is a tube.

6. The apparatus of claim 4 including, means forming a fluid cooledjacket surrounding said radiator means to shield certain of the interiorsurfaces of the vacuum system from heat radiated outwardly from saidradiator means.

7. The apparatus of claim 2 wherein said getter advancing meansautomaticaly advances the getter material into said sublimation zone ata controlled rate.

8. The apparatus of claim 7 including, means for sensing the gaspressure Within the vacuum system and for controlling, in response tothe gas pressure being sensed, the rate at which the getter material isautomatically advanced into the sublimation zone.

9. The apparatus of claim 5 wherein said sublimation zone is disposedinside said radiator tube, wherein the getter material is advanced intosaid radiator tube from one end thereof with sublimed getter materialetfusing out the other end of said radiator tube, and means disposedover the effusion end of said radiator tube for directing the eifusinggetter material in directions away from the longitudinal axis of saidradiator tube.

10. The apparatus of claim 2 including in combination, means forming achamber to be evacuated by the sublimation pump apparatus, means formingWall portions within said chamber means having surfaces for collecting asurface film of the sublimed getter material for gettering gas coming inconact therewith, means for cooling said film collecting wall portionsto at least liquid nitrogen temperature, and means for ionizing gasWithin said chamber means and for bombarding getter material with theionized gas for pumping the ionized gas.

References Cited UNITED STATES PATENTS 3,244,969 4/1966 Herb et al 324333,313,474 4/1967 Hamilton 230-69 ROBERT M. WALKER, Primary Examiner.

1. A SUBLIMATION VACUUM PUMP APPARATUS OF THE TYPE HAVING A SOURCE OFGETTER MATERIAL WHICH IS SUBLIMED ONTO INTERIOR SURFACES OF A VACUUMSYSTEM FOR GETTERING AND THUS PUMPING GASES WITHIN THE SYSTEM TO BEEVACUATED INCLUDING, MEANS FOR FORMING A THERMAL RADIATOR DISPOSEDADJACENT THE SOURCE OF GETTER MATERIAL FOR PRODUCING A SUBLIMATION ZONEHAVING A TEMPERATURE IN OPERATION ABOVE THE SUBLIMATION TEMPERATURE OFTHE GETTER MATERIAL FOR SUBLIMING THE GETTER MATERIAL IN SAID ZONE, ANDMEANS FOR DIRECTING A STREAM OF ELECTRONS ONTO SAID RADIATOR MEANS FORHEATING SAID RADIATOR TO ITS OPERATING TEMPERATURE.